1. Field of the Invention
The present invention relates to a semiconductor device to be coupled with a control circuit by a photocoupler, and more particularly, it relates to a technique of preventing a malfunction of the semiconductor device caused by external noise.
2. Description of the prior Art
A semiconductor device having a power semiconductor element such as a triode AC switch (TRIAC) or a thyristor is employed under high voltage, in order to generate high power. On the other hand, a control signal to be supplied to a control electrode of the power semiconductor element is generated by a control circuit which operates under low voltage. Therefore, the power semiconductor element and the control circuit are often coupled with each other through a photocoupler, in order to transfer the control signal from the control circuit to the power semiconductor element while maintaining electric insulability between the same. Such a semiconductor device is called "solid-state relay".
In such a semiconductor device coupled with the control circuit through the photocoupler, a malfunction may be caused by external noise, due to a structural character of the semiconductor device. FIGS. 1 and 2 are a sectional view and a circuit diagram showing a conventional solid-state relay, respectively, provided for clear understanding of such a problem. A semiconductor device (solid-state relay) shown in FIG. 2 is provided with a TRIAC 7 serving as a power semiconductor element. The TRIAC 7 has a first main electrode T.sub.1, a second main electrode T.sub.2 and a gate electrode A control signal for the TRIAC 7 is generated in a control circuit (not shown), to be supplied to input terminals 3 and 4. This control signal is supplied through a resistor 8 to a light emitting diode 5a, which in turn emits light. The light emitting diode 5a and a bidirectional photothyristor 5b form a TRIAC coupler (photocoupler) The bidirectional photothyristor 5b is connected between the first and second main electrodes T.sub.1 and T.sub.2 of the TRIAC 7.
On the other hand, a load 18 and an AC power source 16 are connected between the first and second main electrodes T.sub.1 and T.sub.2 through output terminals 1 and 2, as shown in FIG. 3. A resistor 10 is interposed between the main electrode T.sub.1 and the gate electrode G.
Therefore, when light is applied to the bidirectional photothyristor 5b from the light emitting diode 5a, current flows through the bidirectional photothyristor 5b and the resistor 10. The TRIAC 7 is turned on by a voltage drop thus developed at the resister 10. When voltage supplied between the main electrodes T.sub.1 and T.sub.2 from the AC power source 16 is lowered, the TRIAC 7 is turned off. A series combination of a capacitor 8 and a resistor is interposed between the main electrodes T.sub.1 and T.sub.2, in order to absorb surge current.
Within the aforementioned elements, the terminals 1 to 4, the TRIAC coupler 5 and the TRIAC 7 are shown in FIG. while the remaining ones are not shown in FIG. 1. The semiconductor circuit shown in FIG. 2 is mounted on an insulating substrate. The insulating substrate 11 is prepared by a ceramic substrate or a printed board, for example, and is mounted on the upper surface of a metal plate 12 for heat radiation. The semiconductor circuit having the aforementioned elements is contained in a case 13, and sealed by a resin member 14.
The TRIAC 7 generates a considerable amount of heat in operation. The heat is transferred to the heat radiating metal plate 12 through the insulating substrate 11, to be diffused to a radiator (not shown on which the metal plate 12 is mounted. Therefore, the insulating substrate 11 is preferably as small as possible in thickness, so far as electric insulability is ensured between the semiconductor circuit and the heat radiating metal plate 12. Also in case of employing an insulated metal substrate formed by integrating the metal plate 12 and the insulating substrate an insulation layer corresponding to the insulating substrate 11 is preferably small in thickness. Thickness of the insulating substrate is selected within a range of 100 to 500 .mu.m, for example.
When the insulating substrate 11 is thus small in thickness, a capacitor C.sub.S is caused between the semiconductor circuit and the metal plate 12, as shown in FIG. 3. If external noise 17, which is symbolically shown as a noise source in FIG. is applied between a ground level and the semiconductor circuit, current flows through the resistor 10 and the capacitor C.sub.S, whereby voltage is developed across the resistor 10. Due to such voltage, current flows between the gate electrode G and the first main electrode T.sub.1 through the interior of the TRIAC 7. This current functions as a trigger signal for the TRIAC 7, whereby the TRIAC 7 is undesirably turned on. As a result, the semiconductor device malfunctions by the external noise.